This invention relates to cable fault locating apparatuses, and more particularly to such an apparatus employing a bridge circuit.
A Murray bridge for locating a fault caused in a cable is well known in the art. In the Murray bridge, a loop circuit is formed by a faulty cable, the fault point to be located, a sound return cable, and a ratio arm connected between the two ends of the cables. A bridge source is connected between the output terminal of the ratio arm and the ground, and a galvanometer is connected between two terminals of the ratio arm.
In order to operate at high sensitivity with this Murray bridge type cable fault locating apparatus, it is necessary to provide an electrical source as high in voltage as several thousands of volts so as to supply necessary current to the bridge through the ground and the high resistance of the faulty point. Therefore, in the case of using the Murray bridge type cable faulty locating apparatus, before performing the bridge balancing operation thereof, it is necessary to carry out a so-called burnout operation in which the faulty point is burnt out by using another electric source, thereby decreasing the resistance of the faulty point, or the measurement is conducted with low sensitivity.
In order to overcome the above-described difficulty and to perform bridge work under the condition of high fault resistance with an appropriate sensitivity of galvanometer, a so-called inverted Murray bridge has been proposed which is obtained by replacing the position of the bridge source and the position of the galvanometer by each other in the aforementioned Murray bridge. In the inverted Murray bridge, it is possible to employ a low voltage source of the order of 1.5 to 2.0 volts and a galvanometer high in current sensitivity. Accordingly, the sensitivity of the inverted Murray bridge is, as a whole, much higher than that of the conventional Murray bridge. However, the inverted Murray bridge is still disadvantageous in that before initiating the bridge balancing operation, a stray earth current may be allowed to flow into the galvanometer, as a result of which the needle of the galvanometer is deflected out of the scale. It is therefore frequently impossible to carry out the detection of the faulty point with the apparatus employing the inverted Murray bridge. For this reason, the use of the inverted Murray bridge for locating a fault in a cable is impractical.